Allow customization of the initial frame name.

Rather than hardcode the use of "main" as the initial frame's name, use `persp-initial-frame-name` to allow a user defined string.
(perp-initial-frame-name): Fixed Docstring
(persp-show-modestring): Change to defcustom

Set as choice to allow choice between off (nil), modeline (t) and header ('header).
Add defcustom for `persp-modestring` formatting

(persp-modestring-dividers): New defcustom to allow customizing of the modestring markers (opening, closing and dividing text)
(persp-update-modestring): Use values from new defcustom rather than hardcoded values.
Merge pull request #20 from jleechpe/master

Allow customization of the initial frame name.
Merge remote-tracking branch 'jleechpe/custom-modeline-format'

Conflicts:
	perspective.el